It is known that trichlorosilane and silicon tetrachloride form in varying proportions in the hydrochlorination of silicon, depending upon the conditions under which the reaction is performed. An important factor which determines the proportion is the reaction temperature: low reaction temperatures promote the formation of trichlorosilane, while at increasing reaction temperatures the percentage of silicon tetrachloride increases, so that, at temperatures above 600.degree. C., the latter is generally present in amounts of more than 80% in the resulting chlorosilane mixture.
It is economically desirable to operate at the highest possible reaction temperatures, because the reaction proceeds more rapidly and better yields are obtained per unit of time and capacity. Thus it has been observed that in the hydrochlorination of silicon, that is, in the reaction of hydrogen chloride with silicon, in the fluidized bed raising the reaction temperature from 300.degree. to 500.degree. C. results in an increase in the rate of reaction by a factor of 7. When the reaction temperature is raised from 300.degree. to 700.degree. C. the increase amounts to a factor of 22.
For the production of trichlorosilane as the main product, the fluidized bed process at temperatures of 300.degree. to 400.degree. C. is today generally used. In this method a good thermal transfer to the cooled reactor wall is assured, and thus local hot spots in the reaction bed, which lead to a poorer selectivity with respect to trichlorosilane, are avoided. If, however, silicon tetrachloride, is desired as the main product, the reaction takes place in a fixed bed reactor at temperatures of about 1000.degree. C. and higher. Because of the poor thermal transfer in this method, a uniform temperature cannot be maintained in the reactor bed.
A disadvantage of all of the prior art methods is the fact that larger amounts of trichlorosilane are obtained only at relatively low reaction temperatures, at which a slow rate of reaction, and hence a less favorable yield per unit of time and capacity must be reckoned with. Furthermore, prior art methods are not flexible as regards the composition of the chlorosilane mixture, if the reaction temperature is not varied over wide ranges. Varying the reaction temperature over wide ranges to increase flexibility with regard to the trichlorosilane content in the chlorosilane mixture, however, is technically complex and uneconomical in the known methods. In the case of a possible combined production of trichlorosilane and silicon tetrachloride, it becomes very important economically to adapt to a fluctuating demand for the individual components contained in the chlorosilane mixture.
DE-AS 19 42 280 discloses a method for the preparation of chlorosilane by reacting silicon with hydrogen chloride in which this reaction is performed in the presence of hydrogen, and thus the content of trichlorosilane in the chlorosilane mixture can be increased. However, molar ratios of hydrogen chloride to hydrogen of 1:1 to 1:50 are needed in order to achieve the desired effect, and, with increasing reaction temperatures, larger amounts of hydrogen have to be used. The process therefore has the disadvantage that large volumes of reactants have to be handled, and that the separation of the chlorosilanes from the hydrogen requires a great deal of energy.